Eliminating coherence effects in bright-field laser imaging

ABSTRACT

A method for imaging and an imaging system, the system includes the steps of: (i) scanning a beam of coherent radiation over a surface along a scan axis; (ii) focusing the beam to a spot on the surface, so that the spot has a predetermined dimension along the scan axis; (iii) spreading the beam laterally while scanning the beam, so that the beam covers an area substantially wider than the predetermined dimension in a direction transverse to the scan axis; and (iii) capturing the radiation scattered from the surface while scanning the beam, so as to form an image of the surface.

FIELD OF THE INVENTION

[0001] The present invention relates generally to laser scanningsystems, and specifically to methods and systems for optical inspectionof surfaces based on laser scanning.

BACKGROUND OF THE INVENTION

[0002] In high-resolution imaging, the use of coherent illuminationleads to well-known problems of speckle and loss of resolution. Thisproblem has been studied particularly in the context of microscopy, asdescribed by Born and Wolf in Principles of Optics (Seventh Edition,Cambridge University Press, 1999), in Chapter 10, which is incorporatedherein by reference. On page 597, the authors note that the resolutionof an image taken using coherent illumination is determined by a factorm=NA_(C)/NA_(O), wherein NA_(C) is the numerical aperture of the lensused to focus the illumination onto the image plane (the condenser, inmicroscopy terms), and NA_(O) is the numerical aperture of the imagingobjective. For optimal image resolution, it is desirable that m beroughly in the range between 1 and 1.5.

SUMMARY OF THE INVENTION

[0003] It is an object of some aspects of the present invention toprovide an optical imaging system with both high resolution and highthroughput.

[0004] In laser-based bright-field imaging systems, the laser beam isscanned over a surface being imaged, and the light scattered from thesurface is captured by an electronic image sensor. For high throughput,the laser has the advantage of high brightness, and therefore can givehigh photon flux at the image sensor. High resolution, however, requiresthat coherence effects be avoided. For this purpose, as explained in theBackground of the Invention, it is necessary to focus the laser beamwith a numerical aperture at least as great as that of the objectivethat is used to collect the scattered light. The size of the focal spotof the laser beam is inversely proportional to the numerical aperture.Therefore, when the beam is focused with a high numerical aperture, itis able to scan only very narrow lines, as wide as the focal spotitself. Due to the speed limitations of available scanning devices, theneed to cover the surface with a great many of these very narrow scanlines becomes the limiting factor in the throughput of high-resolutionimaging systems known in the art.

[0005] To overcome this limitation, in embodiments of the presentinvention, the laser spot is spread laterally on the surface, in adirection transverse to the primary scan axis. In some embodiments, thislateral spread is accomplished by rapid transverse scanning of the laserbeam, in a direction perpendicular to the primary scan direction. As aresult, the scan lines described by the laser are effectively broadened,without affecting the speed of the primary scan and while maintaining ahigh numerical aperture in focusing the beam onto the surface.Alternatively, the laser beam may be split into a number ofclosely-spaced spots, mutually spaced in the transverse direction, andthese spots may be scanned together to cover the desired scan area.

[0006] In other embodiments, the high numerical aperture, and hencetight focus, of the laser beam is maintained in the direction of thescan axis, but a lower numerical aperture is used in the directiontransverse to the scan axis. The focal spot of the laser beam on thesurface is therefore broadened in the transverse direction. The scanlines are broadened concomitantly, giving enhanced throughput at theexpense of reduced resolution along the transverse direction.

[0007] The invention provides a method for imaging, including thefollowing steps: (i) scanning a beam of coherent radiation over asurface along a scan axis; (ii) focusing the beam to a spot on thesurface, so that the spot has a predetermined dimension along the scanaxis; (iii) spreading the beam laterally while scanning the beam, sothat the beam covers an area substantially wider than the predetermineddimension in a direction transverse to the scan axis; and (iv) capturingthe radiation scattered from the surface while scanning the beam, so asto form an image of the surface.

[0008] The invention provides an imaging apparatus, that includes: (i) aradiation source, which is adapted to generate a beam of coherentradiation; (ii) a scanner, which is adapted to scan the beam over asurface along multiple parallel scan lines having a predeterminedspacing therebetween at a rate selected so as to traverse apredetermined linear distance on the surface over the course of a firstscan period, each of the scan lines having a scan axis, and which isfurther adapted, during the first scan period, to scan the beamrepetitively in a direction transverse to the scan axis, with a secondscan period substantially shorter than the first scan period, so thatthe beam substantially covers the predetermined spacing between the scanlines; and (iii) optics, which are adapted to focus the beam to a spoton the surface and to collect the radiation that is scattered from thespot, so as to form an image of the surface.

[0009] The invention provides an imaging apparatus, including: (i) aradiation source, which is adapted to generate an input beam of coherentradiation; (ii) a beam-dividing element, which is adapted to divide theinput beam into a plurality of parallel beams; (iii) a scanner, which isadapted to scan the plurality of parallel beams over a surface alongmultiple parallel scan lines having a predetermined spacingtherebetween, each of the scan lines having a scan axis; and (iv)optics, which are adapted to focus the plurality of parallel beams so asto form on the surface an array of spots, which are disposed along adirection transverse to the scan axis, and to collect the radiation thatis scattered from the spots so as to form an image of the surface.

[0010] The invention provides an imaging apparatus, that includes: (i) aradiation source, which is adapted to generate an input beam of coherentradiation; and (ii) scanning optics, which are adapted to scan the beamof coherent radiation over a surface along a scan axis and to focus thebeam to a spot on the surface, so that the spot has a predetermineddimension along the scan axis, while spreading the beam laterally sothat the beam covers an area substantially wider than the predetermineddimension in a direction transverse to the scan axis, the optics beingfurther adapted to capture the radiation scattered from the surfacewhile scanning the beam, so as to form an image of the surface.

[0011] The present invention will be more fully understood from thefollowing detailed description of the embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic side view of a laser-based bright-fieldimaging system, in accordance with a embodiment of the presentinvention;

[0013]FIG. 2 is a schematic, pictorial view of a dual-axis scanner, inaccordance with a embodiment of the present invention;

[0014]FIG. 3 is a schematic top view of a surface scanned by a laserbeam, in accordance with a embodiment of the present invention;

[0015]FIG. 4 is a schematic side view of a laser-based bright-fieldimaging system, in accordance with another embodiment of the presentinvention; and

[0016]FIG. 5 is a schematic top view of a surface scanned by a laserbeam, in accordance with the embodiment of FIG. 4.

DETAILED DESCRIPTION OF EMBODIMENTS

[0017]FIG. 1 is a schematic side view of a system 20 for bright-fieldimaging of a surface 22, in accordance with a embodiment of the presentinvention. Typically, surface 22 is the upper surface of a semiconductorwafer, and system 20 is used to observe and detect defects on thesurface. The wafer is preferably mounted on a translation stage 24,which positions the wafer for inspection. Alternatively, surface 22 maybelong to a substrate or object of substantially any other type that isamenable to bright-field scanning for imaging and/or inspection.

[0018] Surface 22 is illuminated by a beam of coherent radiation,preferably from a laser 26. A scanner 28 deflects the laser beam overthe surface, along a primary scan axis in the X-direction, i.e., in thedirection perpendicular to the page surface in the view shown in FIG. 1.Stage 24 translate surface 22 in steps along the Y-direction, so thatthe surface is traversed by a series of parallel scan lines. In oneembodiment, illustrated below in FIGS. 2 and 3, scanner 28 also adds arapid transverse deflection to the laser beam, in the X-direction, inorder to effectively broaden the scan lines.

[0019] The beam from laser 26 is expanded by a telescope 30, and thenpasses through a beamsplitter 32 to be focused onto surface 22 by anobjective lens 34. The telescope and objective lens together define aneffective numerical aperture of the focused laser beam, NA_(C). Assumingthe optics to be diffraction-limited, the laser beam is accordinglyfocused to a spot 36 on surface 22 whose diameter is approximatelyd=λ/NA_(C). At a wavelength λ of 532 nm, and NA_(C)=0.7, for example,the diameter of spot 36 is therefore about 0.76 μm. The light scatteredfrom spot 36 is collected by objective 34, with a collection numericalaperture NA_(O). The collected light is reflected by beamsplitter 32 toan electronic imaging camera 38. To minimize coherence effects thatreduce the resolution of the image formed by camera 38, system 20 ispreferably designed so that NA_(C) is at least equal to NA_(O), and ismost preferably about 1.5 times NA_(O), as described in the Backgroundof the Invention. At the same time, for optimal resolution, the value ofNA_(O) is preferably kept as large as possible.

[0020] Camera 38 comprises an image sensor 40, preferably acharge-coupled device (CCD) matrix array. Typically, sensor 40 comprisesan array of 2048×20 sensors, each approximately 15×15 μm. The sensorincludes readout electronics capable of reading out the charge stored inthe array elements, preferably at a rate of at least 1 billionpixels/sec. Objective 34 and the optics of camera 38 are preferablydesigned for a magnification of 60×, so that each image pixelcorresponds to an area about 0.25 μm across on surface 22. The focusedlaser spot, as noted above, covers 3 pixels. An image processor 42receives and processes the output of sensor 40 to form a high-resolutionimage of surface 22.

[0021]FIG. 2 is a schematic, pictorial view showing details of scanner28, in accordance with a embodiment of the present invention. Thescanner in this embodiment comprises an electro-optic crystal scanningelement 50, followed by an acousto-optic scanning element 54, typicallywith an intervening turning mirror 52. Acousto-optic element 54 scansthe laser beam in the X-direction, i.e., the primary scan direction. Theangular extent of the scan is typically approximately 5°, with a typicalscan period of 20 μs. This angular scan corresponds to a linear scantraversing about 0.5 mm across surface 22 (depending on the choice oftelescope 30 and objective 34). Alternatively, similar scan parametersmay be obtained using a high-speed galvanometer mirror or rotatingprism, as are known in the art.

[0022] Electro-optic element 50 scans the laser beam in the Y-direction.Typically, the scan extent of the electro-optic element is only about0.0250, corresponding to 5 μm on surface 22, much less than that of theacousto-optic element. On the other hand, the scan period of theelectro-optic element is typically only about 10 ns, much shorter thanthat of scanning elements of other types. A waveform generator 56,preferably under the control of processor 42, generates radio-frequency(RF) waveforms to drive elements 50 and 54 in the desired mutualsynchronization.

[0023]FIG. 3 is a schematic top view of surface 22, showing a scanpattern 62 generated by the scanner shown in FIG. 2, in accordance witha embodiment of the present invention. Pixels 60 of sensor 40 areprojected onto surface 22 in this view, as an aid in visualizing theimage that is generated by system 20 as a result. As noted above, thesize of the focal spot of the laser beam formed on surface 22 isapproximately 0.7 μm, while each pixel 60 corresponds to an image areaof 15 μm on the surface. Thus, in the absence of transverse scanningelement 50, the scan pattern generated on the surface by system 20 wouldbe only about a single pixel in width.

[0024] In the example pictured here, however, the transverse deflectionof the laser beam expands the scan pattern laterally to about sevenpixels in width. Scanning elements 50 and 54 are timed so that element54 advances the laser beam by the equivalent of two pixels along thescan axis during a single scan period of element 50. Given a scan periodof 10 ns for scanning element 50, it can be seen that the laser beamcovers surface 22 at a rate of about 7 pixels/ns. Thus, system 20 isable to take advantage of the full readout rate of camera 38, which istypically 1 billion pixels/sec, as described above, substantiallywithout compromising the high numerical aperture used to focus the laserbeam onto surface 22. Consequently, both the resolution and throughputof system 20 are optimized.

[0025] After completing the scan shown in FIG. 3 over the prescribedscan distance in the X-direction, stage 24 is stepped by a distanceroughly equal to the scan width in the Y-direction, i.e., by about sevenpixels in the example shown here. The scan pattern is then repeateduntil the entire region of interest on surface 22 (which may comprisethe entire surface) has been imaged.

[0026] Whereas FIGS. 2 and 3 shows one specific example of how combinedlongitudinal and transverse scanning can be accomplished, alternativeconfigurations will be apparent to those skilled in the art. Thescanning rates and periods of elements 50 and 54 may be adjusted toaccord with different imaging resolution requirements and differentsizes and speeds of sensor 40. Moreover, in alternative embodiments ofthe present invention (not shown in the figures), the longitudinal andtransverse scanning functions of scanner 28, as exemplified by FIG. 3,may be accomplished by a single scanning element. For example,acousto-optic element 54 may be driven to deflect the laser beam in boththe X- and Y-directions.

[0027]FIG. 4 is a schematic side view of a system 70 for bright-fieldimaging of surface 22, in accordance with another embodiment of thepresent invention. This embodiment is largely similar to that shown inFIG. 1, except for the addition of a beam dividing element 72, whichcreates an array of spots 36 on the surface, mutually spaced in theY-direction. Each of the spots preferably has a small diameter and highnumerical aperture, as described above. Various methods are known in theart for splitting the laser beam into multiple spots. One example is aDamman grating, which separates an incoming beam into multiple orders ofequal power. As another example, an arbitrary beam array may be designedusing methods described by Morrison in U.S. Pat. No. 5,559,724, whosedisclosure is incorporated herein by reference.

[0028]FIG. 5 is a schematic top view of surface 22, showing scanpatterns 80, 82 and 84 generated in system 70, in accordance with aembodiment of the present invention. In this example, the incident laserbeam is split into three equal spots, which are spaced three pixelsapart in the Y-direction. The scan patterns of the three spots aresimilar to pattern 62 (FIG. 3), except that the extent of the lateralscan, in the Y-direction, is substantially reduced. This arrangementreduces the demands on scanner 28 and may enable more rapid overallcoverage of surface 22.

[0029] In another embodiment of the present invention, not shown in thefigures, the laser beam is focused onto the surface with a non-uniformnumerical aperture—high NA in the X-direction, preferably equal to orgreater than NA_(O), and lower in the Y-direction. As a consequence ofthe non-uniform optical configuration, the laser beam forms a focal spotthat is elongated in the Y-direction, transverse to the X-direction scanline. Thus, each scan over the surface covers a wide area, as in theembodiment shown in FIG. 3, without the need for additional transversedeflection of the beam by scanning element 50 or other means. Theresolution of the image will, of course, be compromised in theY-direction, but the full resolution afforded in the X-direction issufficient for some applications.

[0030] Although the embodiments described hereinabove use certainparticular types of optics and optical configurations, the principles ofthe present invention may similarly be implemented in other opticalsystems, using other types of optical components. It will thus beappreciated that the embodiments described above are cited by way ofexample, and that the present invention is not limited to what has beenparticularly shown and described hereinabove. Rather, the scope of thepresent invention includes both combinations and subcombinations of thevarious features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

1. A method for imaging, comprising: scanning a beam of coherent radiation over a surface along a scan axis; focusing the beam to a spot on the surface, so that the spot has a predetermined dimension along the scan axis; spreading the beam laterally while scanning the beam, so that the beam covers an area substantially wider than the predetermined dimension in a direction transverse to the scan axis; and capturing the radiation scattered from the surface while scanning the beam, so as to form an image of the surface.
 2. A method according to claim 1, wherein scanning the beam comprises scanning the beam along multiple parallel scan lines having a predetermined spacing therebetween that is substantially greater than the predetermined dimension, and wherein spreading the beam comprises spreading the beam so as to cover the spacing between the scan lines.
 3. A method according to claim 1, wherein capturing the radiation comprises collecting the radiation using an objective having a predetermined objective numerical aperture, and wherein focusing the beam comprises focusing the beam with a focusing numerical aperture that is approximately equal to or greater than the objective numerical aperture.
 4. A method according to claim 1, wherein spreading the beam laterally comprises scanning the beam in the transverse direction, so as to cover the area substantially wider than the predetermined dimension.
 5. A method according to claim 4, wherein scanning the beam along the scan axis comprises scanning the beam so that the beam traverses a predetermined linear distance on the surface along the axis over a first scan period, and wherein scanning the beam in the transverse direction comprises scanning the beam over the area substantially wider than the predetermined dimension with a second scan period, substantially shorter than the first scan period, so that the beam is repetitively scanned in the transverse direction multiple times during the first scan period.
 6. A method according to claim 1, wherein spreading the beam laterally comprises dividing the beam so as to form on the surface multiple spots of the predetermined dimension, wherein the multiple spots are disposed along the direction transverse to the scan axis.
 7. A method according to claim 6, wherein spreading the beam laterally further comprises scanning the multiple spots over the surface in the transverse direction.
 8. A method according to claim 1, wherein focusing the beam comprises focusing the beam with a first numerical aperture along the scan axis, and wherein spreading the beam laterally comprises focusing the beam with a second numerical aperture, smaller than the first numerical aperture, in the direction transverse to the scan axis.
 9. A method according to claim 1, wherein capturing the radiation comprises forming a bright field image of the surface.
 10. Imaging apparatus, comprising: a radiation source, which is adapted to generate a beam of coherent radiation; a scanner, which is adapted to scan the beam over a surface along multiple parallel scan lines having a predetermined spacing therebetween at a rate selected so as to traverse a predetermined linear distance on the surface over the course of a first scan period, each of the scan lines having a scan axis, and which is further adapted, during the first scan period, to scan the beam repetitively in a direction transverse to the scan axis, with a second scan period substantially shorter than the first scan period, so that the beam substantially covers the predetermined spacing between the scan lines; and optics, which are adapted to focus the beam to a spot on the surface and to collect the radiation that is scattered from the spot, so as to form an image of the surface.
 11. Apparatus according to claim 10, wherein the optics are adapted to focus the beam to a spot having a predetermined dimension, which is substantially smaller than the spacing between the scan lines.
 12. Apparatus according to claim 11, and comprising an imaging sensor having a predetermined pixel size, and wherein the optics are adapted to image the surface onto the sensor such that the spacing between the scan lines is multiple times the pixel size.
 13. Apparatus according to claim 10, wherein the optics comprise an objective, which is configured to collect the radiation with a predetermined objective numerical aperture, and wherein the optics are further configured to focus the beam with a focusing numerical aperture that is approximately equal to or greater than the objective numerical aperture.
 14. Apparatus according to claim 13, wherein the image comprises a bright-field image, and wherein the optics are configured to focus the beam onto the surface through the objective.
 15. Apparatus according to claim 10, wherein the scanner comprises a first scanning element, which is configured to scan the beam along the scan axis, and a second scanning element, which is configured to deflect the beam in the direction transverse to the scan axis.
 16. Apparatus according to claim 15, wherein the first scanning element comprises an acousto-optic element, while the second scanning element comprises an electro-optic element.
 17. Apparatus according to claim 10, wherein the optics comprise a beam-dividing element, which is adapted to divide the beam so as to form on the surface multiple spots, which are disposed along the direction transverse to the scan axis, and wherein the scanner causes the multiple spots to scan together over the surface.
 18. Imaging apparatus, comprising: a radiation source, which is adapted to generate an input beam of coherent radiation; a beam-dividing element, which is adapted to divide the input beam into a plurality of parallel beams; a scanner, which is adapted to scan the plurality of parallel beams over a surface along multiple parallel scan lines having a predetermined spacing therebetween, each of the scan lines having a scan axis; and optics, which are adapted to focus the plurality of parallel beams so as to form on the surface an array of spots, which are disposed along a direction transverse to the scan axis, and to collect the radiation that is scattered from the spots so as to form an image of the surface.
 19. Apparatus according to claim 18, wherein the optics are adapted to focus the plurality of parallel beams so that the spots have a predetermined dimension, which is substantially smaller than the spacing between the scan lines.
 20. Apparatus according to claim 19, and comprising an imaging sensor having a predetermined pixel size, and wherein the optics are adapted to image the surface onto the sensor such that the spacing between the scan lines is multiple times the pixel size.
 21. Apparatus according to claim 18, wherein the optics comprise an objective, which is configured to collect the radiation with a predetermined objective numerical aperture, and wherein the optics are further configured to focus the plurality of parallel beams with a focusing numerical aperture that is approximately equal to or greater than the objective numerical aperture.
 22. Apparatus according to claim 21, wherein the image comprises a bright-field image, and wherein the optics are configured to focus the beam onto the surface through the objective.
 23. Apparatus according to claim 18, wherein the scanner comprises a first scanning element, which is configured to scan the plurality of parallel beams along the scan axis, and a second scanning element, which is configured to deflect the beams in the direction transverse to the scan axis.
 24. Apparatus according to claim 23, wherein the first scanning element comprises an acousto-optic element, while the second scanning element comprises an electro-optic element.
 25. Imaging apparatus, comprising: a radiation source, which is adapted to generate an input beam of coherent radiation; and scanning optics, which are adapted to scan the beam of coherent radiation over a surface along a scan axis and to focus the beam to a spot on the surface, so that the spot has a predetermined dimension along the scan axis, while spreading the beam laterally so that the beam covers an area substantially wider than the predetermined dimension in a direction transverse to the scan axis, the optics being further adapted to capture the radiation scattered from the surface while scanning the beam, so as to form an image of the surface.
 26. Apparatus according to claim 25, wherein the scanning optics are adapted to scan the beam along multiple parallel scan lines having a predetermined spacing therebetween that is substantially greater than the predetermined dimension, and to spread the beam so as to cover the spacing between the scan lines.
 27. Apparatus according to claim 25, wherein the scanning optics comprise an objective having a predetermined objective numerical aperture for capturing the scattered radiation, and wherein the imaging optics are adapted to focus the beam to the spot on the surface with a focusing numerical aperture that is approximately equal to or greater than the objective numerical aperture.
 28. Apparatus according to claim 25, wherein the scanning optics are adapted to scan the beam in the transverse direction so as to cover the area substantially wider than the predetermined dimension.
 29. Apparatus according to claim 25, wherein the scanning optics comprise a beam dividing element, which is adapted to divide the beam so as to form on the surface multiple spots, which are disposed along the transverse direction. 